The role of immunological processes in the etiology of chronic periodontal disease is of current interest. Immunologic factors are also suggested to be operative in developing acquired resistance to dental caries. Utilizing the gnotobiotic rat as a model system, these problems will be approached from three aspects. First, using organisms demonstrated to be of key interest in the etiology of oral disease in rats, data will be provided on the immune responses to monoinfection. Systemic responses will be emphasized including quantitative measurements of immunoglobulin classes, specific antibody levels, lymph node and spleen morphology, and cellular responses. Systemic responses will be compared with local manifestations of the immune response in saliva and oral tissues. The second major objective is to consider the role of the complement (C) system in relation to oral disease. Recent data has shown the bioactive role of C in certain periodontal disease processes. Immunofluorescent techniques and complment intermediates will be employed in this phase of study. The present of C and selected C components in saliva, serum, normal and diseased tissues will be investigated. The third phase of study involves polyinfection of gnotobiotic rats with selected oral pathogens, and other pertinent organisms including C1. difficile. The effects of the order of introduction of different species in a sequence of contaminations will be investigated. Similarly the survival of implants of certain organisms during conventionalization will be considered. Results will be interpreted in terms of disease, growth of the species, and comparative immune responses. This information should be of value in better understanding the differences in composition of oral flora within individuals of the same species.